Structural adhesives are adhesive compositions that can bond materials with a mechanical strength comparable to mechanical fasteners. They may be used to replace or augment conventional joining techniques such as welding, brazing or mechanical fasteners, such as nuts and bolts, screws and rivets. In particular, in the transportation and construction industries, structural adhesives can present a light weight support of or even an alternative to mechanical fasteners.
Epoxy resin based compositions have been long known for their good adhesive and mechanical properties and have been widely used as bonding agents in a variety of applications. Many of these compositions contain latent curatives (for example dicyandiamides, anhydrides or aromatic amines, such as for example diaminodiphenyl sulfone) and require high temperatures for curing the adhesive composition. Such adhesive systems are referred to as “one-component systems”. Other epoxy adhesive formulations with more reactive curing agents can be cured at lower temperatures. Such systems are referred to as “two-component systems”, because at least the majority of the epoxy resins are kept separated from the curing agents to avoid premature cross-linking. The two parts are combined upon application of the adhesive to initiate the curing reaction.
Small parts or objects, such as e.g. brackets, are frequently used for manufacturing operations in construction, automotive, aeronautics or aerospace industries. Small parts such as brackets are generally adhesively fixed within the interior of aircrafts so as to provide e.g. supporting means for cables. Due to the multitude of these small parts, the adhesive compositions used for their adhesive bonding shall provide specific performance requirements such as room temperature and fast curing, mainly because these small parts generally have to be applied manually.
Examples of room-temperature curable structural adhesive compositions used for adhesive bonding operations in aeronautic and aerospace industries are described e.g. in EP 2 402 394 (Wu et al.).
In manufacturing operations for construction, automotive, aeronautics or aerospace industries, it is often required that small parts such as brackets are capable of being adhesively bonded to surfaces which are inclined, vertical or overhead. Known structural adhesive compositions which are used for such bonding operations of small parts are typically used in combination with supporting fixtures, whereas the supporting fixture is meant to temporarily hold in place the assembly consisting of the small part and the adhesive composition until the later exhibits an acceptable adhesive strength. The supporting fixtures used in combination with existing adhesive compositions are for one application and are therefore disposed of after usage.
Without contesting the technical advantages associated with the adhesive compositions known in the art for bonding small parts such as brackets, there is still a strong need for rapid curing adhesive curable compositions suitable for use in structural bonding applications, which can advantageously replace the known compositions and obviate the need to use additional supporting fixtures during the adhesive bonding operations.
Other advantages of the structural adhesives and methods of the invention will be apparent from the following description.